(1) Field of the Invention
The present invention relates to a crystal oscillation circuit, and more particularly to a crystal oscillation circuit which is served to oscillate on the basis of an oscillation frequency of a crystal oscillator.
(2) Description of the Related Art
An oscillation circuit provided with a crystal oscillator is widely used in various appliances such as a watch, a cellular phone and a computer terminal because the oscillation circuit is stable in frequency. For these portable electronic appliances., the life of a cell built therein is a significant performance characteristic. Hence, the lower power consumption of the crystal oscillation circuit that is being constantly operated in such an appliance has been an important technical issue.
In order to lower the power consumption of the crystal oscillation circuit, conventionally, a constant voltage generator dedicated to the crystal oscillation circuit has been provided so that a constant electric power may be supplied to an oscillating amplifier located in the circuit, for reducing the power consumption of the crystal oscillation circuit. (Refer to the Official Gazettes of Japanese Unexamined Patent Publication No. 10-213686 (pages 8–9 and FIG. 2), Japanese Unexamined Patent Publication No. 04-94201 (page 3 and FIG. 1), Japanese Unexamined Patent Publication No. 06-59756 (page 3 and FIG. 2), and Japanese Unexamined Patent Publication No. 2002-359524 (page 3 and FIG. 1).)
FIG. 10 shows the conventional crystal oscillation circuit. As shown, the crystal oscillation circuit is arranged to have a PMOS transistor M101, an NMOS transistor M102, a resistor R101, a crystal oscillator X101, capacitors C101 and C102, and a constant voltage generator 101.
The constant voltage generator 101 is composed of an operational amplifier Z101. The operational amplifier Z101 is supplied with a voltage sent from a power supply Vdd. The operational amplifier Z101 is inputted with a reference voltage Vref at its positive phase input terminal. Further, the operational amplifier Z101 supplies a constant voltage Vreg. The constant voltage generator 101 operates to supply the transistors M101 and M102 with a constant voltage Vreg of e.g. 2 V to 1.2 V. In general, the constant voltage generator 101 composes a voltage follower circuit through the use of the operational amplifier Z101 as shown in FIG. 10.
The capacitors C101 and C102 compose a positive feedback circuit with the crystal oscillator X101. The transistors M101 and M102 are served as an oscillating amplifier (oscillating inverter) for exciting an exciter composed of the crystal oscillator X101. The resistor R101 is a feedback resistor for specifying the operating points of the transistors M101 and M102 served as the oscillating inverter.
The transistors M101 and M102, the capacitors C101 and C102, and the crystal oscillator X101 compose an oscillation circuit, which oscillates the oscillation voltages ampin and ampout at a resonant frequency substantially determined by the crystal oscillator X101. In order to keep the oscillation, the transistors M101 and M102 are indispensable because those transistors amplify the oscillation voltage ampin and compensate for a loss. caused by the crystal oscillator X101. The transistors M101 and M102 are supplied with a constant voltage Vreg sent from the constant voltage generator 101 and drive the capacitors C101 and C102 and the crystal oscillator X101 with the supplied constant voltage Vreg. The constant voltage Vreg to be supplied to the transistors M101 and M102 is specified so that the constant voltage Vreg may be large enough to keep the oscillation but small enough not to make an idle passage current too large.
The constant voltage Vreg is generated by the voltage follower circuit (operational amplifier Z101) being inputted with the reference voltage Vref as shown in FIG. 10. The voltage follower circuit generates a constant voltage Vreg of e.g. 2 V to 1.2 V without depending on the voltage value of the power supply Vdd and then supplies the constant voltage to the transistors M101 and M102. This voltage follower circuit therefore causes the crystal oscillation circuit to keep its oscillation and the transistors M101 and M102 to avoid idle power consumption.
FIG. 11 illustrates an operational amplifier composing the voltage follower shown in FIG. 10. As shown in FIG. 11, the operational amplifier composing the voltage follower is made up of PMOS transistors M103, M104 and M107, NMOS transistors M105, M106, M108 and M109, and capacitors C103 and C104. The transistor M105 is inputted with the reference voltage Vref. The sources of the transistors M103, M104 and M107 are inputted with the voltage of the power supply Vdd. The transistor M107 outputs the constant voltage Vreg at its drain. The gates of the transistors M108 and M109 are inputted with a bias voltage NB101.
The connection of a minus input terminal (reverse phase input terminal) of a circuit generally called a two-stage operational amplifier as shown in FIG. 11 with an output electric potential (the specification of the gate potential of the transistor M106 into the constant voltage Vreg) composes the voltage follower, so that the constant voltage Vreg is substantially equal to the reference voltage Vref. Giving the reference voltage Vref a voltage of e.g. 2 V to 1.2 V without depending on the voltage value of the power supply Vdd, the voltage follower generates a constant voltage Vreg of e.g. 2 V to 1.2 V without depending on the voltage value of the power supply Vdd. Since the two-stage operational amplifier has a large gain, the constant voltage Vreg is accurately matched to the reference voltage Vref.
The use of the foregoing voltage follower has made it possible to lower the power consumption of the crystal oscillation circuit.
Moreover, as an alternative means, the supply of constant current to an oscillating inverter leads to reducing the power consumption of a crystal oscillation circuit. (For example, refer to the Official Gazettes of Japanese Unexamined Patent Publication No. 07-7325 (page 5 and FIG. 2), Japanese Unexamined Patent Publication No. 11-150419 (page 3 and FIG. 1), Japanese Unexamined Patent Publication No. 2002-359524 (page 7 and FIG. 4), Japanese Unexamined Patent Publication No. 11-150420 (page 3 and FIG. 1), and Japanese Unexamined Patent Publication No. 2004-177646 (page 2 and FIG. 1)). FIG. 12 illustrates the second conventional crystal oscillation circuit. As shown, the crystal oscillation circuit is arranged to have PMOS transistors M110, M112 and M113, NMOS transistors M111, a depletion NMOS transistor M114, a resistor R102, a crystal oscillator X102, and capacitors C105 and C106. The transistors M110 and M111 compose an oscillating inverter.
FIG. 13 illustrates the third conventional crystal oscillation circuit. In the crystal oscillation circuit shown in FIG. 13, the same components as those shown in FIG. 12 have the same reference numbers and are not described herein. As shown in FIG. 13, the transistor M110 composing the oscillating inverter is connected with the transistor M115 that supplies current.
FIG. 14 illustrates the fourth conventional crystal oscillation circuit. In the crystal oscillation circuit shown in FIG. 14, the same components as those shown in FIG. 12 have the same reference numbers and are not described herein. As shown in FIG. 14, the gates of the transistors M110 and M111 composing the oscillating inverter are connected with the capacitors C107 and C108 respectively. Further, a resistor R103 is connected between the gate and the drain of the transistor M111. Moreover, the constant voltage output circuit composed of PMOS transistors M116 and M117, NMOS transistors M118 and M119, and a resistor R105 supplies a bias voltage to the gate of the transistor M110 through a resistor R104.
In FIGS. 12 to 14, the functions of the transistors M110 and M111, the capacitors C105 and C106 and the crystal oscillator X102, all of which compose the oscillating inverter, are the same as those of the crystal oscillation circuit shown in FIG. 10. Hence, the description about the function of those components is left out. Then, the description will be oriented to the supply of current to the transistors M110 and M111.
In the crystal oscillation circuit shown in FIG. 12, the current defined by the depletion NMOS transistor N114 is supplied to the transistors M110 and M111 composing the oscillating inverter so that the inverter may be operated. In the crystal oscillation circuit shown in FIG. 13, keeping the gate potential of the transistor M115 at the ground GND (of the power supply Vdd), current is supplied to the transistors M110 and M111 composing the oscillating inverter so that the inverter may be operated. In the crystal oscillation circuit shown in FIG. 14, given a bias voltage to the gate of the transistor M110 composing the oscillating inverter through the resistor R104, the current flowing through the transistors M110 and M111 is controlled so that the inverter may be operated.
As described above, the supply of constant current to the oscillating inverter has led to lowering the power consumption of the crystal oscillation circuit.
In the meantime, with recent reduction of electronic appliances in size, a request for improving performance of portable electronic appliances is growing more and more. As described above, for the portable electronic appliances or a watch, the life of a cell built therein is a significant performance characteristic. Hence, the lower power consumption of the crystal oscillation circuit that is constantly operated in these appliances becomes a more and more important technical issue.
However, in the case of using the two-stage operational amplifier as shown in FIG. 11 as the voltage follower, it is necessary to design a frequency characteristic of a negative feedback loop so as not to make the operation of the voltage follower unstable due to its oscillation. For that purpose, the capacitor for phase compensation is used for the two-stage operational amplifier. In general, this type of capacitor needs a disadvantageously large capacitance and circuit area.
Concretely, as shown in FIG. 11, it is necessary to provide the capacitor C103 at the first stage output (the drains of the transistors M103 and M105) of the two-stage operational amplifier and at the second stage output (the drains of the transistors M107 and M109). This allows a dominant pole to be sufficiently kept away a second pole on a frequency axis, resulting in being able to ensure enough phase margin even in the case of driving a capacitive load (capacitor C104).
In a case that the load connected with the drains of the transistors M107 and M109 are capacitive, the capacitor C103 for phase compensation is required to be provided according to the capacitance of the load. However, in a case that the load capacitance is small or no stabilization capacitance is specially provided for the constant voltage Vreg, the provision of the phase compensation capacitor may be unnecessary. That is, in a case that the load capacitance is large, the capacitance of the capacitor C103 is required to be large accordingly. On the other hand, in order to prevent increase of an occupation area, it is considered that the capacitance of the capacitor C104 for suppressing transient fluctuation of the constant voltage Vreg is suppressed to be lower. In this case, however, if the constant voltage Vreg is greatly fluctuated, it is necessary to make the set potential high enough to keep the oscillation at the constant voltage Vreg dropped by the larger transient fluctuation. This results in increasing the power consumption of the crystal oscillation circuit.
Further, in the method of supplying constant current to the oscillating inverter for reducing the circuit power, in the circuit shown in FIG. 12, since the current defined in the depletion NMOS transistor M114 is supplied to the transistors M110 and M111, the current to be supplied is likely to be adversely influenced by the manufacturing variation of the transistor M114 and thus is not stable. Moreover, in the CMOS process, it is necessary to prepare the depletion MOS transistor that is not commonly available. This was a problem.
In FIG. 13, keeping the gate potential of the transistor M115 at the GND causes the transistors M110 and M111 composing the oscillating inverter to be inputted with current from the power supply Vdd. This current depends on the voltage of the power supply Vdd and thus is made smaller if the voltage of the power supply Vdd is low. Hence, if the circuit is designed so that the oscillation may be kept even at a lower voltage of the power supply Vdd, the current is made larger than required if the voltage of the power supply Vdd is high and moreover the current is likely to be adversely influenced by the manufacturing variation of the transistor M115. This was a problem as well.
In the circuit shown in FIG. 14, the use of a circuit having a thermal voltage as its reference (a circuit composed of the transistors M116 to M119 and the resistor R105) for a circuit for supplying a bias voltage to the transistor M110 of the oscillating inverter circuit leads to improving dependency of the bias current on the power supply. Further, the bias circuit is not so much influenced by the manufacturing variation of the MOS transistor. However, the supply of the bias voltage for controlling current to the gate of the transistor M110 composing the oscillating inverter keeps the output amplitudes of the transistors M110 and M111 at the voltage of the power supply Vdd. Hence, for controlling the signal amplitude, it is necessary to supply the constant voltage of the power supply Vdd. That is, in the circuit shown in FIG. 14, though the circuit for overcoming the dependency of the bias voltage on the supply voltage is used, another circuit for generating a voltage is required for controlling the signal amplitude. These two circuits disadvantageously make the overall oscillation circuit complicated.